Cerebral and cerebellar cortices, choroid plexus and spinal cord of mice, inoculated intracerebrally with a brain emulsion containing Japanese encephalitis virus (JEV), were studied electronmicroscopically to determine the cell type and the site of JEV replication. 72 hours after inoculation, when the mice began to show encephalitic symptoms, 70 to 80% of all cortical neurons and anterior horn cells contained many spherical particles mostly located in the smooth endoplasmic reticulum and a few in the granular endoplasmic reticulum. The individual particles demonstrated an uniform substructure consisting of an electron dense central core of 25–30 mμ diameter, an outer less electron dense zone and an outermost limiting membrane of 40 mμ diameter. 96 hours after inoculation, the cytoplasm of cortical neurons and anterior horn cells was observed to contain very many vacuoles and vesicles. Particles were found widely scattered throughout the vacuoles and vesicles, and were observed for the first time in the endoplasmic reticulum of the stellate neurons and in Purkinje cells, though fewer. No such particles were observed in control and normal mouse groups. So-called eosinophic intranuclear inclusions of epithelial cells of choroid plexus failed to show any particles in their nuclei or cytoplasmic vesicles. Considering that no particulate matter, identifiable as JEV, was identified within any of the glial cells or endothelium in this examination, it was concluded that JEV was really neurotropic and replicates in the endoplasmic reticulum of the neurons.